Aircraft measurements over Europe of an air pollution plume from Southeast Asia – aerosol and chemical characterization

[摘要] An air pollution plume from Southern and Eastern Asia, including regions inIndia and China, was predicted by the FLEXPART particle dispersion model toarrive in the upper troposphere over Europe on 24–25 March 2006. According tothe model, the plume was exported from Southeast Asia six days earlier,transported into the upper troposphere by a warm conveyor belt, and travelledto Europe in a fast zonal flow. This is confirmed by the retrievals of carbonmonoxide (CO) from AIRS satellite measurements, which are in excellentagreement with the model results over the entire transport history. Theresearch aircraft DLR Falcon was sent into this plume west of Spain on 24March and over Southern Europe on 25 March. On both days, the pollution plumewas found close to the predicted locations and, thus, the measurements takenallowed the first detailed characterization of the aerosol content andchemical composition of an anthropogenic pollution plume after a nearlyhemispheric transport event. The mixing ratios of CO, reactive nitrogen(NO_y) and ozone (O₃) measured in the Asian plume were all clearlyelevated over a background that was itself likely elevated by Asianemissions: CO by 17–34 ppbv on average (maximum 60 ppbv) and O₃ by2–9 ppbv (maximum 22 ppbv). Positive correlations existed between thesespecies, and a ΔO₃/ΔCO slope of 0.25 shows that ozone wasformed in this plume, albeit with moderate efficiency. Nucleation mode andAitken particles were suppressed in the Asian plume, whereas accumulationmode aerosols were strongly elevated and correlated with CO. The suppressionof the nucleation mode was likely due to the large pre-existing aerosolsurface of the transported larger particles. Super-micron particles, likelydesert dust, were found in part of the Asian pollution plume and also insurrounding cleaner air. The aerosol light absorption coefficient wasenhanced in the plume (average values for individual plume encounters0.25–0.70 Mm⁻¹), as was the fraction of non-volatile Aitken particles.This indicates that black carbon (BC) was an important aerosol component.During the flight on 25 March, which took place on the rear of a troughlocated over Europe, a mixture of Asian pollution and stratospheric air wasfound. Asian pollution was mixing into the lower stratosphere, andstratospheric air was mixing into the pollution plume in the troposphere.Turbulence was encountered by the aircraft in the mixing regions, where thethermal stability was low and Richardson numbers were below 0.2. The resultof the mixing can clearly be seen in the trace gas data, which are followingmixing lines in correlation plots. This mixing with stratospheric air islikely very typical of Asian air pollution, which is often lifted to theupper troposphere and, thus, transported in the vicinity of stratosphericair.

[学科分类] 大气科学